Two former Parramatta Eels players are accused of harbouring semi-automatic weapons and possessing more than half-a-million dollars in cash after dramatic arrests in Sydney's Centennial Park yesterday.

It was a weekend where personal responsibility went out the window, as wild brawls engulfed the central night-time entertainment precincts of Sydney and Melbourne.

Saturday night has now been dubbed "fight night" in Sydney, with five hours of violent carnage being scoured by police.

The most serious event took place at 3am in Sydney's CBD, where revellers from a fundraising event at the Metro Theatre spilled onto the streets.

As fists began to fly and a brawl escalated, officers from three separate police stations were called in to control the situation, in which 30 men and women involved.

"Police took strong action and we've arrested a number of people, and we'll continue to try and arrest more people who were involved in that," NSW Police Assistant Commissioner Mick Fuller said.

It was the most brutal in a string of incidents, including a number of scuffles in the CBD and the Rocks, as well as a fight over a late night meal in Randwick.

A brazen and confronting attack also occurred on a Sydney bus, after a man assaulted a passenger. He fled the scene, but was arrested by police a short time later. It is not clear whether he was affected by drugs or alcohol.

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"Alcohol plays a role in most violence across the state, regardless of whether that's domestic violence or violence in the street," Mr Fuller said.

"It's something where people need to take responsibility for their actions and clearly, from what we saw over the weekend, they didn't take responsibility for actions. But in those cases we will."

The Saturday night violence in Sydney followed an already violent Friday night.

Two weeks out from her wedding, Lauren Estabolio, 29, was celebrating her hens night when she found herself in the back of an ambulance.

Miss Estabolio and her friends were approached by a man allegedly making sexual advances towards them. When they resisted, his friend attacked - punching and kicking the women.

"These men certainly attacked these women, it was unprovoked, and thankfully we've arrested both of these individuals," Mr Fuller said.

Three of the women were taken to St Vincent's Hospital. Miss Estabolio was the most seriously injured, with a broken collarbone.

"It was just the ideal for people drinking too much and we saw lots of people drinking during the day, more than we would normally see, and we just saw all the accidents, fights, falls through the whole weekend," St Vincent's head of emergency Gordian Fulde said.

But Sydney wasn't the only city to experience a wild weekend of violence.

The heart of Melbourne was turned into a war zone, with hundreds of rival gang members descending on the CBD.

The city was filled with families, fresh from a day spent celebrating the annual Moomba Festival.

By 8pm, more than 150 teenagers with links to notorious suburban gangs descended on Flinders Street Station.

Police were outnumbered 10-to-one as the teenage thugs began their violent rampage, catching officers completely off-guard.

Chairs were hurled like missiles, as families ran for their lives.

A kebab shop owner was caught up in the rampage, with CCTV capturing the moment police swoop on one teenager trying to fight officers as frightened customers hid inside the store.

Victoria Police deputy commissioner Andrew Crisp says a new taskforce has been set up to crack down on the gang violence, with detectives today seizing and sifting through hours of security footage.

"The footage that we have is clear," Deputy Commissioner Crisp said.

"I'd suggest that you should actually give yourself up, don't wait for us to come knocking at your door," he warned.

"It's also pretty dumb, isn't it, when we have the world's most sophisticated CCTV identifying every face and every person."

Melbourne Lord Mayor Robert Doyle has echoed the sentiment from police.

"This will not define the way we use and enjoy our city," Mr Doyle said.